1. Field of the Invention
This invention relates to a distributed feedback semiconductor laser device, the absorption loss of which has been adjusted to the same periodicity as the wavelength of laser light in the direction of the propagation of the laser light, thereby allowing oscillation in a single longitudinal mode.
2. Description of the Prior Art
It has been found in recent years that distributed feedback semiconductor laser devices, the refractive index of which has been changed to match the periodicity of the wavelength of laser light in the direction of the propagation of the laser light, are an effective means for laser light oscillation in a single longitudinal mode. In a semiconductor laser with a distributed feedback the refractive index of which has been changed, there are essentially two longitudinal modes with equivalent gain. However, when uniform electrical current is injected into the device, that is, when device operation is under a steady state, oscillation is attained in only one longitudinal mode because of some asymmetrical characteristics of the resonator.
In most semiconductor laser devices with distributed feedback, the facets of both ends of the devices do not result in those of a Fabry-Perot resonator, but rather, one of the facets is etched diagonally, so that the asymmetry of the resonator is increased greatly, and stable single-mode oscillation can be achieved.
However, when the above-mentioned conventional semiconductor lasers with distributed feedback operate under an unsteady state where the excitation current is rapidly modulated as when they are being used as a light source for optical communication, the above-mentioned asymmetry of the resonator is probably changing constantly. Thus, oscillation is not necessarily in a single mode, and in practice it involves broadening of the oscillation spectrum and noncontinuous change in the oscillation mode.